Illuminated graphic displays and buttons for automotive applications such as radios often have backlit insignia which identify the particular function of the display or button. Such backlit components have a light source which is positioned behind the insignia in order to make the insignia visible in the dark, necessitating that the insignia be capable of transmitting light from the light source. However, backlit components must also be visible in daylight, necessitating that the insignia also be capable of reflecting light available within the passenger compartment.
A known process for manufacturing buttons and other backlit components is the use of paint and laser technology. These processes have generally involved the use of a transparent plastic substrate which is painted white to form a white translucent layer over the transparent substrate, and then painted black to form an opaque black covering over the white translucent layer. The black covering is then lased away to expose a portion of the white translucent layer, which serves as the insignia. The transparent nature of the substrate maximizes the transmission of light through the backlit component for night time viewing, while the white translucent layer contributes graphics whiteness by reflecting light, such that the insignia can be visible under natural lighting conditions during daylight hours. As used herein, the term "graphics whiteness" is employed in accordance with industry standards to quantitatively and qualitatively describe the level of light reflected by a surface. A standard known in the art and employed herein for evaluating graphics whiteness is the color space Y value per the international standard 1931 CIE (Commission International de l'Eclairage).
A variation of the above structure is disclosed in U.S. Pat. No. 4,729,067 to Ohe. Ohe teaches the use of a transparent substrate over which is essentially deposited a translucent layer and a light diffusing layer. The transparent substrate is an acrylic resin, while the light diffusing layer is preferably composed of an acrylic resin matrix in which is dispersed a light diffusing agent. The translucent layer serves to bond the light diffusing layer to the transparent substrate, and enhance the diffusion of the light transmitted through the substrate into the light diffusing layer. However, the layers are delineated by chemically reacted surfaces, making the utilization of the teachings of Ohe rather complicated and expensive for mass production.
Another variation of the more conventional structure described earlier is disclosed in U.S. Pat. No. 3,694,945 to Detiker. Detiker teaches the use of a white translucent substrate over which is formed an opaque grating composed of an opaque reflective layer and a covering layer. The translucent substrate is formed from silicate glass or a polymer, such as an acrylic resin or polycarbonate, while the reflective layer is composed of metal or an acrylic resin lacquer in which a metal is dispersed. The covering layer may be of any suitably translucent material, such as a lacquer. The reflective layer serves to prevent light emitted from a light source beneath the substrate from reaching the covering layer, and then reflects the light back toward the substrate. Consequently, light emitted by the light source escapes only through openings in the grate. In essence, backlighting of a display formed in accordance with Detiker is transmitted through a translucent substrate, and not a transparent substrate and translucent layer. However, generating a grate in accordance with Detiker is relatively expensive and limits the use of such techniques to relatively large displays.
The paint and laser process described earlier also has significant shortcomings. Insignias typically used in automobile graphic displays have a stroke width (the line width of the insignia) of only about 0.5 millimeter. Obtaining suitable optical characteristics with such intricate graphics requires very tight control of the cured thickness of the white paint in order to maintain the desired reflectance and transmissive properties. Often, as a result of the limitations of paint processes and paint chemistry, the thickness of the white paint must be maintained between about 20 and about 30 micrometers in order to achieve suitable lighting intensities for daytime and nighttime viewing. However, the variation in thickness between backlit components within a display group must be maintained within a .+-.2.5 micrometer range in order to provide a uniform lighting appearance.
Furthermore, different backlighting intensities of adjacent insignia result in irregular illumination intensities within the display group. This is particularly true with buttons of a backlit display which share one or more light sources. To minimize costs, such groupings often use a minimum number of light sources, and incorporate light pipes for the purpose of distributing the light energy equally to each of the backlit components. Though much effort has been directed toward optimizing the capability of light pipes, uniform backlighting of each and every backlit component is very difficult due to size and location restraints. As a result, facets and painted patterns have often been applied to light pipes in order to increase the light intensity directed to relatively dim areas. In particular, reflectors and additional lamps have been required, while excessively bright areas have been attenuated with printed halftone patterns behind the individual insignia.
While such tactics have been effective for flat screen printed displays, it is very costly and poorly suited for buttons and other backlit components which are not flat and have low lighting intensities. Generally, from a production standpoint, it is most cost effective if all the buttons for a given display group are molded in a single mold and subsequently finished as a set. Deviations from this approach typically have not been cost effective or practical. An example is the use of different shades of white paint on buttons within a single display group. Other approaches, such as molding each button from a white translucent substrate whose thickness is specifically tailored to achieve acceptable backlighting intensities, have resulted in unacceptable graphics whiteness of the insignia under natural lighting conditions. In addition, molding the substrate of adjacent buttons from materials with different light transmission characteristics is not feasible in a production environment.
From the above, it can be seen that the prior art lacks a method by which differing light transmission and reflection characteristics can be suitably provided for backlit components in order to equalize their backlighting intensities and reflectivities. Accordingly, it would be desirable if a process existed by which a group of non-flat molded plastic backlit components could be manufactured with minimal variability in backlighting intensity. Such a method would allow each backlit component to be individually tailored to exhibit a suitable level of backlighting intensity when backlit by a minimal number of light sources. Furthermore, such a method would produce backlit components whose reflection characteristics under daylight conditions also yielded a suitable and uniform level of graphics whiteness.